Nucleating agents have been hitherto employed in silver halide photographic materials for various purposes. For instance, hydrazines, which have been most frequently employed as nucleating agents, have been used as nucleus forming agents in internal latent image type direct positive silver halide emulsions and also have been used for the purpose of increasing sensitivity and/or gradation in negative type surface latent image forming silver halide emulsions.
Silver halide emulsions in which light-sensitive nuclei are mainly present in the interior of silver halide grains and wherein latent images are mainly formed in the interior portion of the grains are called internal latent image type silver halide emulsions, as distinguished from silver halide grains which mainly form latent images on the surface of the grains.
Methods for obtaining direct positive images upon surface development of an internal latent image type silver halide photographic emulsion in the presence of a nucleating agent, and photographic emulsions or light-sensitive materials to be used for such methods are generally known, and described, for example, in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, 3,227,552 and 3,317,322, British Patents 1,011,062, 1,151,363, 1,269,640 and 2,011,391, Japanese Patent Publication Nos. 29405/68 and 38164/74, Japanese Patent Application (OPI) Nos. 16623/78, 137133/78, 37732/79, 40629/79, 74536/79, 74729/79, 52055/80 and 90940/80 (the term "OPI" as used herein means an "unexamined published application"), etc.
In the above described methods for obtaining direct positive images, while nucleating agents may be added to a developing solution, it is more common to add the nucleating agents to photographic emulsion layers or other appropriate layers of light-sensitive materials.
Hydrazine compounds are most widely known as nucleating agents which are added to silver halide photographic light-sensitive materials. In general, hydrazine type nucleating agents are excellent as to some important properties, such as sensitivity discirmination, since they provide a large difference between maximum density (Dmax) and minimum density (Dmin). However, they are disadvantageous because they require processing at a high pH (pH&gt;12).
Nucleating agents which function in processing at a low pH (pH.ltoreq.12) are known, however, such as heterocyclic quaternary ammonium salts described, for example, in U.S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738, 3,759,901, 3,854,956, 4,094,683 and 4,306,016, British Patent 1,283,835, Japanese Patent Application (OPI) Nos. 3426/77 and 69613/77, etc. Particularly, propargyl- or butynyl-substituted heterocyclic quaternary ammonium salt compounds as described in U.S. Pat. No. 4,115,122 are excellent nucleating agents in view of sensitivity discrimination when used in direct positive silver halide emulsions. However, in silver halide emulsions, particularly those for color photographic light-sensitive materials, sensitizing dyes are usually employed for the purpose of spectral sensitization. In such cases, competitive adsorption of the sensitizing dyes and the heterocyclic quaternary ammonium type nucleating agents onto silver halide grains takes place, and thus, it is necessary to add a large amount of the quaternary ammonium salt type nucleating agents which are of low adsorptivity. In particular, in the case of multilayer color photographic light-sensitive materials, unevenness of density and poor color balance may undesirably occur. Therefore, these compounds are still insufficient.
In order to resolve this problem, quaternary salt type nucleating agents having a silver halide adsorption accelerating thioamido group are disclosed in U.S. Pat. No. 4,471,044. Although the amount to be added necessary to obtain a sufficiently high Dmax is reduced and the decrease in Dmax during preservation at high temperature is controlled by introduction of such an adsorptive group, these effects still do not achieve a fully satisfactory level.
Further, hemicyanine type quaternary salt compounds are described in Japanese Patent Application (OPI) No. 100426/82. However, these compounds also do not achieve a fully sufficient level of nucleating activity.
On the other hand, it is also known that quaternary ammonium salt type compounds may accelerate development in silver halide negative emulsions as described, for example, in U.S. Pat. No. 4,135,931, Japanese Patent Application (OPI) Nos. 114328/77 and 121321/77, German Patent 2,647,940, Belgian Patent 721,568, etc.